(1) Field of the Invention
The present invention relates to a gas ejection duct with acoustic treatment, and to an aircraft having such a duct. The invention also relates to a method of fabricating such a duct.
The invention lies in the technical field of gas ejection ducts for aircraft.
(2) Description of Related Art
An aircraft usually has an engine contributing to providing the aircraft with propulsion or indeed lift. For example, a rotorcraft may have a fuel burning engine that rotates a rotor contributing at least to providing the aircraft with lift.
The engine generates exhaust gas that is expelled out from the aircraft by an ejection system. On an aircraft, a gas ejection system may include a duct commonly referred to as a “nozzle” for discharging exhaust gas out from the aircraft. An engine may also include a gas takeoff duct taking gas to other equipment of the aircraft.
The engines used in aviation can produce noise that is painful for a human to hear. Gas ejection systems are therefore sometimes acoustically treated in an attempt to reduce the noise they emit, in particular in a frequency range that is painful to hear for the human ear.
The gas ejection systems of aircraft also need to satisfy mechanical strength requirements relating to the surrounding environment and to satisfy the requirements of official aviation regulatory agencies.
In order to reduce emitted noise, a mechanical system may have walls provided with small cells that are locally open to the outside and that form so-called “Helmholtz” resonators. Such cells are referred to as “cavities” for convenience.
Helmholtz resonance is a physical phenomenon that appears when a sound wave travels close to a cavity in communication with the outside via an opening. The original sound wave penetrates into the cavity through the opening, and is then reflected inside the cavity so as to form a wave that is phase shifted, while remaining an image of the original acoustic wave. The original acoustic wave, when superposed on the phase shifted wave, leads to a resulting wave that is attenuated. The level of sound emission is thus decreased.
Such cavities therefore operate in the presence of acoustic waves such as a vibrating system having one degree of freedom with its mass associated with the dimensions of the cavities, with its stiffness associated with the volume of air in each cavity, and with its damping associated with the resistance opposed by the outside medium to generating acoustic waves.
On an aircraft, a wall provided with resonators making use of this principle has a core defining small cavities of hexagonal section in application of the principle of the Helmholtz resonator. Such a cavity is then in the form of a prism of hexagonal section extending in elevation from an open base towards an open top. Given this shape, the core is referred to as a “honeycomb” layer.
The core is then brazed to a rear metal sheet that closes the base of each prism. Furthermore, a perforated skin rests on the tops of the prism so that each cavity can communicate via a plurality of perforations with the ejected gas.
Thus, a helicopter gas ejection prototype with acoustic treatment includes sheets made of titanium. Sheets are shaped and welded together to form a wall having cavities of hexagonal section or cavities of large dimensions. The wall also has a perforated skin attached to the cavities.
That gas ejection prototype provides cavities presenting dimensions that are different. As a result, the use of cavities with different volumes serves to attenuate noise over a wider frequency range. Nevertheless, making different cavities turns out to be difficult when using metal sheets that are brazed to one another.
In addition, assembling sheets requires numerous fabrication operations. In particular, such assembly can require fine metal sheets to be brazed together, which is difficult.
Consequently, making a gas ejection duct, and in particular a nozzle, that is provided with such acoustic treatment means can be difficult and/or expensive.
Documents FR 2 712 640, FR 2 929 336, and EP 2 865 947 are remote from the problem of the invention and are mentioned purely by way of information.
Document FR 2 712 640 describes a structure having a surface with low aerodynamic drag. That structure has cells opening out to an outside medium via one opening and to a channel via another opening.
Document FR 2 929 336 relates to a device having plane jets used to reduce the noise generated by an aircraft jet engine. The device has a wall surrounding a stream of gas ejected by a jet engine. Ducts are distributed at a periphery of a downstream end of the wall in order to eject a jet of fluid in the form of a sheet.
Document EP 2 865 947 provides a damper in order to reduce acoustic pulsations in a gas turbine chamber. That damper has a cavity opening out to a bent tube in communication with the chamber.
Documents EP 1 998 003, U.S. Pat. No. 4,240,252, U.S. Pat. No. 4,452,335, WO 2015/098148, and EP 1 391 597 are also known.